RequestAdapter blocks forever on macos

[thisguycodes]

The compute example doesn't work on macos, it hangs forever on instance.RequestAdapter(nil), using delve it looks like the callback never fires.

You have tests that would cover this, but it looks like CI only runs a limited subset of tests.

[kolkov]

Thanks for the report — confirmed. We've seen the same behavior with the Rust backend on macOS ARM64 in the gogpu ecosystem (gogpu/gg#171).

The issue is in how we handle the async callback from wgpu-native — likely CallbackModeAllowProcessEvents + ProcessEvents() doesn't work on macOS Metal. Working on a fix.

[kolkov]

We've done a deep investigation. Here's what we found and a diagnostic request.

Analysis

wgpu-native calls the adapter callback synchronously inside wgpuInstanceRequestAdapter() — the CallbackMode field is ignored entirely. On Windows this works because syscall.LazyProc.Call handles re-entrant callbacks natively. On macOS, our FFI layer uses a different call path (runtime.cgocall → assembly), and the synchronous callback may not fire correctly.

Two possible causes:

1. Metal main thread requirement — MTLCopyAllDevices() blocks because it's not on the main thread
2. FFI re-entrant callback — the C function returns but the callback doesn't fire (Go→C→Go re-entrancy issue)

Diagnostic

We've pushed a debug branch with prints before/after the C call and inside the callback handler. Could you test it?

# In your project, use the debug branch:
go get github.com/go-webgpu/webgpu@debug/macos-hang
go mod tidy

# Run your code (or the compute example):
DYLD_LIBRARY_PATH=/path/to/wgpu-native go run .

Expected output patterns:

Case A — C function blocks (Metal thread issue):

[wgpu-debug] calling wgpuInstanceRequestAdapter...
(hangs here — never prints "returned")

Case B — Callback doesn't fire (FFI re-entrancy issue):

[wgpu-debug] calling wgpuInstanceRequestAdapter...
[wgpu-debug] wgpuInstanceRequestAdapter returned
[wgpu-debug] callback did NOT fire synchronously, entering ProcessEvents loop
(spins forever)

Case C — Working (what we see on Windows):

[wgpu-debug] calling wgpuInstanceRequestAdapter...
[wgpu-debug] adapterCallbackHandler FIRED: status=0 adapter=0x... userdata1=1
[wgpu-debug] wgpuInstanceRequestAdapter returned
[wgpu-debug] callback fired synchronously (before loop)

Also: are you on Apple Silicon (arm64) or Intel (amd64)?

[ivokub]

I tested on MacOS 26.4 with both wgpu-native@master (compiled from source) and v29.0.1. I have the case B:

Init... OK
CreateInstance... OK
[wgpu-debug] calling wgpuInstanceRequestAdapter...
[wgpu-debug] adapterCallbackHandler FIRED: status=1 adapter=0x600003218470 userdata1=0
[wgpu-debug] wgpuInstanceRequestAdapter returned
[wgpu-debug] callback did NOT fire synchronously, entering ProcessEvents loop